1. Field of the Invention
The present invention relates to a carbon monoxide (CO) absorbing liquid. More particularly, the present invention is directed to a CO absorbing liquid which can absorb CO at a high speed with a large absorption quantity and can be used for a long time with a high oxygen resistance.
2. Description of the Prior Art
CO is contained in exhaust gases discharged not only from iron manufacturing plants but also from other various plants or in product gases in various plants. CO can be used as a fuel or as a starting material in the synthetic chemical industry, but CO is poisonous. Various processes for separating CO from exhaust or product gases have been proposed for utilization or removal of CO.
As the conventional CO-separating process, there can be mentioned a cryogenic method utilizing the difference in the boiling points between respective gases, an absorption method using a liquid capable of absorbing CO and a conversion method which comprises reacting CO with hydrogen or water in the presence of a catalyst to convert CO to methane or hydrogen, respectively.
Each of these conventional CO-separating processes involves certain defects as described below and thus a process which can continuously separate CO by a simple operation has not yet been developed.
In the cryogenic method, the treatment should be conducted at an extremely low temperature. In the conversion method, since the reaction is carried out at a high temperature, in the presence of a catalyst, the equipment and operation become complicated. The absorption method is simple because separation is accomplished by causing the gas to be treated to come in contact with an absorbing liquid so that CO can be absorbed in the absorbing liquid. However, CO absorbing liquid heretofore used for the absorption of CO are greatly influenced by the copresence of gases such as oxygen and carbon dioxide and also they cannot be used for a long time. Therefore, the absorption method has not been broadly adopted.
Recently, there has been proposed a so-called liquid film method as a novel gas separation technique for separating CO.sub.2 from a gas mixture containing CO.sub.2 and O.sub.2 or separating hydrogen sulfide from gasified coal. This method is diagrammatically illustrated in FIG. 1. A protecting film 2 and a porous plate 3 are placed on both surfaces of a hydrophilic film 1, such as a hydrophilic polypropylene film impregnated with an absorbing liquid. A gas containing a component to be separated, such as CO, is caused to make contact with one surface of the liquid film 1 whereby CO is absorbed in the liquid film 1, and the other surface of the liquid film is subjected to pressure reduction or caused to make contact with another gas such as He, to desorb absorbed CO.
In the liquid film method, since absorption and desorption of CO can be performed simultaneously, a continuous gas treatment is possible. The liquid film method is also advantageous in that the required energy is small and the intended gas can be selectively separated.
In this liquid film method, properties of the liquid film especially the absorbing property, are obviously important. More specifically, the absorbing liquid that is used in the liquid film method should have a large gas absorption quantity and a high absorbing speed, and the absorbing liquid should be stable against other gases which may be present and degradation should not be caused by the copresence of said gases.
Also, in this newly proposed liquid film method, as in the above-mentioned absorbing liquid method, the absorbing liquid is very important, and if a good CO absorbing liquid is developed, it is expected that this absorbing liquid will be used broadly for the separation of CO in not only the liquid film method but also the absorbing liquid method.
Various CO absorbing liquids have heretofore been proposed. For example, in the CO separation method called the "COSORB method," which is disclosed in the specification of U.S. Pat. No. 3,651,159, a solution formed by dissolving CuAlCl.sub.4 in toluene is used as the CO absorbing liquid. Although this absorbing liquid is advantageous in that the quantity of CO that can be absorbed is large, degradation by water is serious and this absorbing liquid can hardly be used for the treatment of gases containing water.
Further, a solution of an ammonia complex salt of cuprous chloride (CuCl), called an "ammonical CuCl solution" and a hydrochloric acid solution of CuCl, called "hydrochloric acid-CuCl," have been proposed as the CO absorbing liquid. Solutions containing copper ions are advantageous in that the CO absorption quantity is large. However, the ammoniacal CuCl solution is defective in that CO.sub.2 or oxygen is absorbed in a large quantity and thus the life is short. The hydrochloric acid-CuCl solution is defective in that the cuprous ions are oxidized and the CO-absorbing capacity is degraded. In order to prevent this degradation of the CO-absorbing capacity, SnCl.sub.2 is ordinarily added to the hydrochloric acid-CuCl solution, but even if SnCl.sub.2 is added, the speed of absorption of oxygen causing oxidation is not lowered, and therefore, attainment of the effect of prolonging the life of the CO absorbing liquid cannot be expected.